The present invention concerns colorless, highly transparent polyamide mixtures (blends) or alloys with improved transparency, chemical resistance, and slight natural color, as well as a process improving the melting capacity and homogeneity of blends consisting of polyamide according to formulas Ia/Ib and partly crystalline polyamides according to formulas IIa/IIb. On the basis of the compositions according to the invention, transparent polyamide blends are obtained which in terms of transparency, color, chemical resistance, particularly stress cracking resistance against isopropanol, exhibit improved properties without resulting losses in the mechanical properties. At the same time a distinct increase in extrusion throughput could be achieved while maintaining optimum transparency.
Amorphous or partly crystalline polyamides, copolyamides or block co-polyamides are employed in numerous technical applications requiring high chemical resistance, dimensional stability under heat, toughness, and strength. Typical processing shapes are fibers, sheeting, hollow articles, thermoplastic adhesives, and injection-molded technical materials. In numerous cases highly transparent materials are required, as for instance for automotive lights, viewing panels, spectacle frames, optical lenses and glass, filter cups, electrical components, optical wave guides, display windows, sight glasses, wrapping films, bottles and containers. The prominent property is chemical resistance against liquids used in automotive applications.
Suitable properties of homopolyamides, copolyamides, or block copolyamides often are tuned through monomer fractions such as lactams or amino acids, diamines and dicarboxylic acids and produced by processes such as cocondensation. Partly crystalline polyamides often are combined and tuned to the desired property profile in the form of polyamide blends by mixing or alloying in the molten state, and depending on requirements supplemented by reinforcing agents, stabilizers such as phenolic antioxidants and HALS types, dyes, processing aids, flame retardants.
In most cases, transparent amorphous or transparent microcrystalline polyamides, copolyamides or block copolyamides are produced by direct cocondensation, as mentioned earlier, while selecting the individual monomer fractions in such a way that the desired target properties are obtained and crystallization is suppressed or adjusted to a microcrystallite form.
The preparation of transparent polyamide blends is complicated by the fact that one must combine, either miscible or isorefractive polyamides. Polyamides which are not miscible or isorefractive can only be mixed or alloyed to a transparent product if by compatibilizers, high mixing temperatures or forced mechanical mixing at low machine throughputs the particle size of the phases can be reduced to less than the scattering size for visible light. The additional extrusion step causes the products to be more expensive than those of direct condensation, and hence is rarely employed for transparent polyamides.
An improved chemical resistance for instance when desired for amorphous polyamides can be achieved by the known method of cocondensation, for instance by the incorporation of long-chain aliphatic monomers such as amino acids diamines, or dicarboxylic acids having more than six carbon atoms or of lactams having 6 to 12 carbon atoms.
Improved chemical resistance as characterized by reduced swelling in ethanol can be attained by producing blends of amorphous polyamides with partly crystalline polyamides via an extrusion step (see DE 2,642,244 C2).
An optimum stress cracking resistance, for instance in isopropanol, can be attained when a particular melting enthalpy of the partly crystalline component is retained in the blend.
It is a disadvantage that this will alter the basic properties of the amorphous polyamide, and that the transparency is reduced by the dispersed phase that is signaled by the melting enthalpy. It is a further disadvantage that depending on the mixing effect of the forming process, the melting enthalpy will be reduced and thus the chemical resistance in terms of stress cracking resistance will decrease.
JP 60-215,053 and 60-215,054 (Kokai) describe blends of transparent polyamides built up from lactams, amino carboxylic acids, cycloaliphatic diamine (bis-(4-aminocyclohexyl)methane), isophthalic acid, and partly crystalline aliphatic polyamides. It is known that longer residence times (lower throughput) is required in the extruder in order to obtain transparent material from these mixtures. Opaque products are obtained at optimum throughputs. The additional extrusion increases the yellow tinge of the products.
In EP 0,070,001 (Du Pont), blends of partly crystalline and amorphous polyamides are described. These amorphous polyamides in addition contain cycloaliphatic diamines for applications with low humidity uptake and good ZnCl2 resistance. However, these blends are not always transparent. According to EP 0 070 001, phenylphosphinate is employed as heat stabilizer.
According to U.S. Pat. No. 4,404,317 (Du Pont), the temperature of the mass should not be above 300xc2x0 C. when extruding blends of amorphous and partly crystalline polyamides in order to obtain optimum color and low decomposition. No information is given relative to chemical resistance and stress cracking behavior.
EP 550,308 B1 describes that extrusion temperatures higher than 300xc2x0 C. are required in order to obtain transparent polyamide blends from polyamide according to formula (I) in that patent and partly crystalline polyamides, since the miscibility is limited at low temperatures and leads to opaque products. The target products should not exhibit any melting enthalpy of the partly crystalline phase in order to obtain optimum transparency. This high thermal stress leads to distinct discoloration of the products. The improved chemical resistance is reported in terms of reduced swelling tendency in ethanol. The reduced swelling tendency in ethanol is attributed to the fact that the terephthalic acid fraction prevails relative to the isophthalic acid fraction. No information is given as to stress cracking behavior, for instance in isopropanol.
Further, a procedure is described in EP 550,308 B1 according to which phosphorus compounds are used as transamidation catalysts in order to produce a post condensation in the extruder. Optionally, in this case a low-viscosity polyamide precondensate is used and extruded together with a second polyamide. Post condensation in the extruder requires vacuum to be applied during extrusion, and leads to higher viscosity of the blend. This type of post condensation requires low throughputs and long residence times, which has negative consequences for the color. Examples referring to this process variant are not provided. Advantages in terms of chemical resistance or stress cracking behavior are not cited.
EP 0,720,631 B1 describes special phosphorus compounds being used as amidation catalysts that can be added as a master batch to polyamide 66 when producing fibers. Adding this master batch to the polyamide melt while producing fibers leads to fibers having a higher molecular weight and higher viscosity but lower gel fraction. The behavior in amorphous polyamides is not reported. The resistance against chemicals and the stress cracking behavior are not described. A disadvantage of these polyamide blends is the higher viscosity, which impairs the fluidity behavior during the processing by injection molding.
In EP 509,282 B1 and DE 3,821,325 A1, phosphorus compounds are proposed as processing stabilizers improving the color of partly crystalline polyamide compounds. The behavior in transparent and amorphous polyamide blends is not described. No information is given as to chemical resistance and stress cracking behavior.
It is an advantage of the present invention, therefore, to avoid at least some of the above disadvantages of the above.
Surprisingly, it has now been found according to the invention that with the aid of phosphorus compounds added during extrusion to the blend components consisting, for instance, of amorphous and partly crystalline polyamides, one obtains highly transparent polyamide blends with improved stress cracking resistances in isopropanol ( greater than 60 MPa) because of a very low melting enthalpy (of less than 8 J/g; see examples B4, B6), even when working with high machine throughputs enabling short residence times.
Despite short residence times a homogeneous, transparent material is obtained according to the invention which exhibits, neither degradation nor polymerization or discoloration but excellent transparency when using extrusion temperatures of 250 to 330xc2x0 C.
For the blends according to the invention, swelling in ethanol is in the range from about 18 to 20 weight percent, regardless of whether the polyamide I according to formulas Ia/Ib has an excess of terephthalic over isophthalic acid or does not contain any terephthalic acid as the acid component.
Relative to polyamide blends produced without the phosphorus compounds that are employed according to the invention, the stress cracking resistance of the blends according to the invention, for instance in isopropanol, will remain on a high level, even when the PA 12 melting peak of the partly crystalline component limiting the transparency is no longer present (see examples B8, VB9, Table 1).
The chemical resistance when stated in terms of swelling in ethanol reveals no differences between blends with and without phosphorus compounds.
The viscosity of the blends according to the invention remains low while a post condensation which would limit the fluidity in subsequent processing by injection molding does not occur. According to the invention, improved welded joint strengths as compared to blends without phosphorus compounds are found.
The chemical resistance of the blends according to the invention, for instance in terms of stress cracks in isopropanol, is distinctly higher than that of cocondensate produced in a direct way (see example VB1, Table 1) and containing exactly the same components in the same proportions. Blends according to the invention exhibit a distinctly lower swelling in ethanol.
When using phosphorus compounds, it is possible in particular to produce transparent blends which without phosphorus compounds require high extrusion temperatures ( greater than 300xc2x0 C.) in order to attain the desired transparency through sufficient homogenization, or require long residence times with low throughput and thus have a discoloration tendency. With the molding materials according to the invention, the melting and mixing effect in polyamides during compounding is almost doubled, the thermal damage is distinctly reduced. The stabilizing effect is retained during subsequent forming, so that products with distinctly improved properties with respect to transparency and color are generated.
In contrast to blends without phosphorus compounds, the chemical resistance exhibited by the molding materials according to the invention in terms of stress cracking resistance in isopropanol is retained, even when achieving optimum homogeneity, so that the partly crystalline phase no longer exhibits melting enthalpy while the swelling behavior in ethanol remains unchanged.
Additional features and advantages of the present invention will be described in and apparent from the detailed description of the presently preferred embodiments and the figures.